linux-kernel - Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

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Message-ID: <alpine.DEB.2.00.1003231221030.10178@router.home>
Date:	Tue, 23 Mar 2010 12:22:57 -0500 (CDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped
 anonymous pages

On Tue, 23 Mar 2010, Mel Gorman wrote:

> diff --git a/mm/migrate.c b/mm/migrate.c
> index 98eaaf2..6eb1efe 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -603,6 +603,19 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  	 */
>  	if (PageAnon(page)) {
>  		rcu_read_lock();
> +
> +		/*
> +		 * If the page has no mappings any more, just bail. An
> +		 * unmapped anon page is likely to be freed soon but worse,
> +		 * it's possible its anon_vma disappeared between when
> +		 * the page was isolated and when we reached here while
> +		 * the RCU lock was not held
> +		 */
> +		if (!page_mapcount(page)) {
> +			rcu_read_unlock();
> +			goto uncharge;
> +		}
> +
>  		rcu_locked = 1;
>  		anon_vma = page_anon_vma(page);
>  		atomic_inc(&anon_vma->migrate_refcount);

A way to make this simpler would be to move "rcu_locked = 1" before the
if statement and then do

if (!page_mapcount(page))
	goto rcu_unlock;

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